Terephthalic acid is produced by the liquid-phase oxidation reaction of p-phenylene compounds such as p-alkylbenzenes typified by p-xylene, usually by using acetic acid as a solvent, a catalyst such as cobalt or manganese, or a catalyst composed of cobalt or manganese and additionally a promoter such as bromine compounds or acetaldehyde. The liquid-phase oxidation reaction uses acetic acid as a solvent, and the obtained crude terephthalic acid slurry includes, in large amounts, impurities such as 4-carboxybenzaldehyde (4CBA), para-toluic acid (p-TOL) and benzoic acid, or other various coloring impurities. Moreover, the crude terephthalic acid obtained by separating from the crude terephthalic acid slurry also includes these impurities as mixed therein, and thus a considerably higher purification technique is required for the purpose of obtaining high-purity terephthalic acid.
As a method for purifying crude terephthalic acid, there have been known various methods such as the following: crude terephthalic acid is dissolved in acetic acid or water, or mixed solvents of acetic acid and water, at high temperature and under high pressure, and then subjected to a catalytic hydrogenation treatment, a decarbonylation treatment, an oxidation treatment, a recrystallization treatment, or a high-temperature immersion treatment in a slurry condition in which terephthalic acid crystals are partially dissolved. Either in the production of crude terephthalic acid by applying a liquid-phase oxidation reaction, or in the purification of the crude terephthalic acid thus obtained, the operation of separating the terephthalic acid crystals from the dispersion medium is finally required.
Oxidation intermediates such as 4CBA, p-TOL, and benzoic acid, or coloration-causing substances present as impurities in the slurry produced by the oxidation reaction or in the slurry obtained by purification treatment of the crude terephthalic acid are mostly dissolved at high temperature in the slurry dispersion medium; however, when the slurry is cooled to approximately 100° C. to form terephthalic acid crystal-containing slurry, these impurities are incorporated into the terephthalic acid crystals, which makes it difficult to obtain high-purity terephthalic acid.
Accordingly, it is required to separate the dispersion medium under the conditions of high temperature and high pressure, in order to obtain high-purity terephthalic acid from the crude terephthalic acid slurry after the oxidation reaction or the slurry after the purification treatment of the crude terephthalic acid. The most commonly used method for separating the dispersion medium from the terephthalic acid crystal-containing slurry is a centrifugation method; a centrifugation method is widely used for the case of the slurry after the oxidation reaction or the case of the slurry after the purification treatment. The feature of the centrifugation method is such that slurry solution is introduced into a basket rotating at a high speed, the dispersion medium is allowed to overflow from the upper portion of the basket, and the crystals are guided to the bottom of the basket; however, it is known that from the constraints due to the structure and the function of the centrifuge, the continuous operation at high temperature and under high pressure involves some difficulties.
First, because the rinse of crystals during centrifugation or after centrifugation is difficult, the amount of the dispersion medium adhered to the crystals tends to increase; in order to solve this problem, usually adopted is a method in which the centrifuged cakes of the terephthalic acid crystals are again turned into slurry by using a fresh high-temperature solvent. However, the aforementioned method leaves a problem that a plurality of times of separation operations are required to be performed. Moreover, a high speed rotation is performed at high temperature and under high pressure, accordingly the maintenance and the protection of the centrifuge is cumbersome and difficult, the investment for the maintenance and the protection is increased, and thus, the aforementioned method is far from being sophisticated.
As a separation method alternative to a centrifugation method, there has been proposed a dispersion medium replacement apparatus taking advantage of the sedimentation of terephthalic acid crystals due to gravitation. For example, Patent Literature 1 discloses a dispersion media replacement apparatus inside which lateral shelves having a plurality of holes are arranged, and describes the occurrence of the degradation of the efficiency of the replacement due to the channeling or back mixing of the fluid in the apparatus in the case where the apparatus does not have such a structure. In addition, Patent Literature 2 describes the improvement of the replacement performance due to the arrangement of the shelves forming inclined planes in the apparatus.
Patent Literature 3 discloses a method in which acetic acid solvent slurry of terephthalic acid crystals obtained by liquid-phase oxidation of p-alkylbenzene compounds is subjected to a mother liquor replacement with a water solvent slurry, wherein a sedimentary layer of terephthalic acid crystals is formed in the bottom of a mother liquor replacement tower, a stirring blade unit is disposed in the sedimented layer, the stirring blade unit is rotated gently so as to maintain the fluidity in the sedimented layer, the purified terephthalic acid slurry is withdrawn from the bottom portion of the mother liquor replacement tower through a screw conveyor, and additionally replacement water for forming an upward flow of water is fed through a screw conveyor.